1. Field of the Invention
The present invention relates to a high tension ceramic condenser, and especially to an improved high tension ceramic condenser which has an excellent cooling-heating cycle characteristic.
2. Description of Prior Art
Recently, gas circuit breakers having a capacity for handling extra-high voltages and which contain SF.sub.6 gas as an extinction medium have been developed as high power circuit breakers, whereby a need has been created for high tension ceramic condensers of improved reliability to be contained in the gas breakers.
As shown in FIG. 1, the electrode terminal for a conventional high tension ceramic condenser is formed as follows. An electrode layer 2 is formed on each of the parallel surfaces of a ceramic dielectric body 1 formed of barium titanate etc., by printing with a molten metal such as silver or a mixture of silver oxide and frit, or by chemically plating nickel. A metal terminal 3 having a screw made of brass is welded or bonded with a solder 4 or an electroconductive adhesive to the electrode layer 2. All parts except the edge of the terminal are encased with a moulded synthetic resin 5.
In the described structure of the conventional ceramic condenser, the metal terminal is made of brass which is easily machined. However, the brass has an expansion coefficient of 20 - 23 .times. 10.sup.-.sup.6 /deg, while the ceramic dielectric has an expansion coefficient of 7 - 9 .times. 10.sup.-.sup.6 /deg, whereby the coefficient of expansion of brass is about 2 - 3 times of that of ceramic dielectric. Accordingly, in soldering the metal terminal, a residual compressive stress which tends to contract the material toward its center is formed in the electrode layer and in the solder layer between the ceramic dielectric and the metal terminal during the cooling period when the temperature drops from the melting point of solder to room temperature.
Furthermore, when ceramic condensers are tested by the conventional cooling-heating cycle test at temperatures ranging from -30.degree.C to 90.degree.C, stresses due to the cooling-heating cycle are added to the residual stress existing at room temperature. Accordingly, the difference in expansion coefficients is large and the contraction coefficient, corresponding to the increased residual stress, increases at the interface between the electrode layer and the solder layer. This increase in the contraction coefficient causes a peeling phenomenon to occur so as to form a void.
FIG. 2 illustrates the formation of a void by the described phenomenon. When the cooling-heating cycle test is performed on a ceramic condenser, the compressive stress corresponding to the deviation between the expansion coefficient and the contraction coefficient at low temperatures is added to the residual compressive stress. Accordingly, as shown in FIG. 2, a void 6 is caused by the peeling phenomenon at the interface between the ceramic dielectric 1 and a portion of the metal terminal of the electrode 2. As shown in FIG. 3, the void is equivalent to connecting an air condenser having a small capacity Cc in series with electrostatic capacity Cs of the ceramic dielectric. When a voltage E(V) is applied, the voltage EQU (Cs/Cc + Cs) .times. E (V)
is applied to the capacitor Cc. As Cc &lt;&lt; Cs, most of the applied voltage is partially applied to Cc. However, the insulation of the small void condenser is quite low whereby a partial discharge is caused upon applying a low voltage, and an apparent corona starting voltage is created as the high tension ceramic condenser is caused to decrease. Accordingly, the reliability of the associated circuit breaker may sometimes be decreased quite seriously because of the described deterioration in the characteristics of the conventional high tension ceramic condenser.